The present invention is concerned with mercury cadmium telluride semiconductor devices. In particular, the present invention is directed to a method of introducing acceptor impurities into mercury cadmium telluride. For the purposes of this specification, the common chemical equations for mercury cadmium telluride, (Hg,Cd)Te or Hg.sub.1.sub.-.sub.x Cd.sub.x Te, will be used.
Mercury cadmium telluride is an intrinsic photodetector material which consists of a mixture of cadmium telluride, a wide gap semiconductor (E.sub.g = 1.6 Ev), with mercury cadmium telluride, which is a semimetal having a negative energy gap of about -0.3 Ev. The energy gap of the alloy varies approximately linearly with x, the mole fraction of cadmium telluride in the alloy. By properly selecting x, it is possible to obtain (Hg,Cd)Te detector material having a peak response over a wide range of infrared wavelengths. High performance (Hg,Cd)Te detectors have been achieved for wavelengths from about 1 to 30 microns.
Mercury cadmium telluride photodiodes have found increasing use in recent years. With this increasing use, more sophisticated photodiodes such as (Hg,Cd)Te reach-through avalanche photodiodes have become desirable. As a result, improved methods of forming PN junctions in (Hg,Cd)Te have become highly desirable.
The formation of PN junctions in (Hg,Cd)Te is complicated by the small dissociation energy of mercury telluride in the alloy. The formation of PN junctions must not cause excessive dissociation of the mercury telluride, since this will adversely affect the electrical and optical properties of the resulting device.
Several techniques have been developed for forming n-type layers on a p-type body of (Hg,Cd)Te. Among these techniques are bombardment with protons, electrons, or mercury ions. These techniques create an n-type layer by creating a damage-induced donor state. These techniques are described in Foyt et al., "Type Conversion and n-p Junction Formation in Hg.sub.1.sub.-x Cd.sub.x Te Produced by Proton Bombardment", Appl. Phys. Let., 18, 321 (1971); Melngailis et al., "Electron Radiation Damage and Annealing of Hg.sub.1.sub.-x Cd.sub.x Te at Low Temperatures", J. Appl. Phys., 44, 2647 (1973); and Fiorito et al., "Hg-Implanted Hg.sub.1.sub.-x Cd.sub.x Te Infrared Photovoltaic Detectors in the 8- to 14-.mu.m Range", Appl, Phys. Let., 23, 448 (1973).
Another technique of forming n-type layers on p-type (Hg,Cd)Te is described by Marine et al., "Infrared Photovoltaic Detectors From Ion-Implanted Cd.sub.x Hg.sub.1.sub.-x Te", Appl. Phys. Let., 23, 450 (1973). This method involves aluminum ion implantation and a subsequent anneal at 300.degree. C for 1 hour to form an n-type region in a p-type (Hg,Cd)Te body.
Techniques for forming a p-type layers on n-type (Hg,Cd)Te, however, are not as well developed. On common method of forming p-type regions in n-type (Hg,Cd)Te is by depositing a gold layer on a surface of the n-type body and then heating the body to diffuse the gold, thereby forming a region of p-type conductivity. This method is described in U.S. Pat. No. 3,743,553 by M.W. Scott et al. While this method is generally satisfactory, it does have some shortcomings. In particular, it is difficult to form very abrupt, well defined PN junctions because gold diffuses extremely rapidly in (Hg,Cd)Te. Devices such as reach-through avalanche photodiodes and wide bandwidth photodiodes, therefore, are difficult, if not impossible, to fabricate using gold diffusion.